The casings for underwater devices, in particular torpedoes, must be possess a variety of characteristics, including: resistance to high pressures, water-tightness and being light.
In the military field, if the casing forms part of a torpedo, the required characteristics are even more stringent. For example, torpedoes are required to resist very high pressures in order to follow modern submarines that can reach increasingly deep operating depths. Torpedoes must be light in order to improve their performance in terms of speed, operating range and systems that can be installed on board.
The use of composite materials, especially carbon fibre, has been proposed for making these casings, as such materials have high mechanical resistance combined with low density. These materials can therefore advantageously replace those that are currently and which are mainly constituted by aluminum alloys.
The coupling between different portions of the casing is carried out using annular metal portions (typically in aluminum or aluminum alloys) that are connected to the portion made of carbon fibre. The connection between the aluminum and carbon fibre parts is quite critical and has limited many practical applications of carbon fibre in making torpedo casings.
According to the known art, the annular portions are provided with a complete conical wall on which an end portion of the carbon fibre casing engages; this stratagem allows creating a large adhesion surface between the conical surface in carbon fibre and the conical surface in aluminum.
Chinese patents CN203528747U and CN103482014 illustrate precisely the use of conical coupling surfaces.
This solution responds very well in many load situations, but is found to be very weak regarding axial loads generated by the pressure acting on the torpedo. Due to the axial loads, the conical surfaces tend to interpenetrate, generating circumferential mechanical loads that can damage the casing. The direct coupling of the carbon fibre on aluminum can also result in corrosion of the aluminum due to a series of chemical reactions between the different materials that, among other things, are both electrical conductors.